Method for pressing particulate material



June 3, 1969 H. MOSS ET AL 3,448,184

r METHOD FOR PRESSING PARTICULATE. MATERIAL Filed April a, 196e UnitedStates Patent O 3,448,184 METHOD FOR PRESSIN G PARTICULATE MATERIALHerbert Irwin Moss, Yardley, Pa., and William Porter Stollar, Edison,NJ., assignors to Radio Corporation of America, a corporation ofDelaware Filed Apr. 8, 1966, Ser. No. 541,325 Int. Cl. B29f 5/02 U.S.Cl. 264-125 5 Claims ABSTRACT F THE DISCLOSURE This invention relates toa method for pressing particulate materials, and particularly, to amethod for compacting powders into unitary bodies. Such methods may,e.g., be used in the manufacture of polycrystalline ferrite bars,infrared transmission Windows and semiconductor bodies.

To obtain uniformly dense compacted masses of powders by pressing, highpressures are essential. Present pressing procedures utilize a die andram assembly in which the die is comprised of a support sleeve and a dieliner made of a high lubricity material, such as graphite. The materialto be pressed is placed in the die liner and compressed between tworams. One diiiiculty with present pressing procedures is mechanicalfailure of the die liner at relatively low pressures. This failurecauses the die liner to crumble and to be forced out of its supportsleeve resulting in the loss of the material being compressed.

An object of this invention is to provide a new and improved method forpressing solid materials.

Another object of this invention is to provide a method capable ofpressing solid materials at greater than normal pressures.

A further object is to provide a method of pressing materials in a dieusing graphite liners at pressures well beyond that which graphiteliners normally fail.

An additional object is to provide a method of compacting and sinteringpowders of inorganic compounds.

The novel method may be used in pressing techniques in which a quantityof powder is pressed with a ram or rams in a die comprised of a dieliner within a support sleeve. The support sleeve provides support forthe die liner in the radial direction, the radial direction being normalto the direction of movement of the rams in the die.

One feature characterizing the novel method is that axial pressure isapplied to the die liner while the material is being pressed within thedie liner. As used herein, the axial direction is parallel to the`direction of movement of the rams in the die. By applying axialpressure to the die liner while pressing the material within the liner,forces are created which reduce the stress within the liner, therebyallowing much higher pressures to be applied to the material to becompressed before the die fails.

The invention is described below in more detail with reference to thedrawings in which:

FIG. l is a front elevational, partially sectional view of a prior-artdie and ram assembly illustrating some of the forces on the die liner.

FIG. 2 is a front elevational, partially sectional view of a die and ramassembly that may be used in practicing the novel method disclosedherein.

Similar reference numerals are used for similar structural elements inthe drawings.

The prior-art die and ram assembly illustrated in FIG. 1 is comprised ofa die 1 and lower and upper rams 2 and 3 adapted to move Within the die.The die 1 is comprised of a die liner 4 within a support sleeve 5. Thesupport sleeve 5 supports the die liner 4 when outwardly directed radialpressure is present on the liner 4.

In operation, the lower ram 2 is placed in the liner 4. A quantity ofmaterial 6 to be compressed is inserted into the liner 4. The upper ram3 is then lowered into the liner 4 and the rams 2 and 3 are moved towardeach other so as to compress the material 6 between them. Duringcompression of the material 6, the rams exert an applied force F1 onsaid material 6. This applied force F1 creates an outwardly directedessentially radial force F2 within the material 6 which causes stresseson the liner 4. The magnitude of the radial force F2 for a given appliedforce F1 depends on the plasticity of the material 6 to be compressed.When the applied force F1 reaches some critical value, depending on thecomposition and dimensions of the liner 4 and on the material 6 beingcompressed, the liner 4 fails by crumbling in a ring like volume and isforced out of the support sleeve 5. It is believed that this is due to acombination of initial radial compressive failure of the liner 4 due tothe radial force F2, followed by axial tensile failure due to thedevelopment of axial components to the radial force F2. These axialcomponents cause the die liner 4 to separate in the area adjacent thecompacted material 6 thereby forcing the liner out of the support sleeve5. The combination of radial compressive and axial tensile failure beinga catastrophic process.

The novel method essentially overcomes the problem of failure of the dieliner as described above. FIG. 2 is a -view of a die and ram assemblythat may be used to practice the novel method of pressure compactingdisclosed herein. FIG. 2 also indicates certain forces present on thedie during the practice of the novel method. The assembly includes lowerand upper rams 2a and 3a respectively which compress material 6a Withina die 1a comprised of "a die liner 4a Within a support sleeve 5a. Thesupport sleeve 5a has an inwardly projecting lip 7 on the lower portionthereof, an internally threaded upper portion `8 and a centralcylindrical portion 9. The inside diameter of the upper portion 8 isgreater than the inside diameter of the central cylindrical portion 9.The liner 4a has a cylindrical outer surface and tits snugly into thecentral cylindrical portion 9 of the support sleeve 5a and is supportedthereby when an outwardly directed radial force is present on the liner4a. The bottom of the liner 4a rests on the lip 7 of the support sleeve5a and is supported thereby. The liner 4a has an upper portion 10 whichextends above the cylindrical portion 9 and into the area of thethreaded upper portion 8 of the support sleeve 5a.

The assembly includes a cylindrical plug 11 threaded on its outercylindrical surface so as to mate with and screw into the threaded upperportion 8 of the support sleeve 5a. The plug 11 has a vertical centerhole so that the upper ram 3a can t therethrough. The lower ram 2aenters the die liner 4a through the end opposite the plug 11. A heatingcoil 12 around the support sleeve 5a can also be included.

In operation, the plug 11 is screwed into the threaded upper portion 8of the support sleeve 5a so as to create a high compressive axial linear`force F3 on the liner 4a in the axial direction. The axial direction,as pointed out above, is deiined herein as the direction in which therams 2a and 3a move in the die liner 4a. The lower ram 2a is thenpositioned in the liner 4a, and the material 6a to be compressed isplaced in the liner 4a. The upper ram 3a is then lowered into place anda compressive force F1 is applied to the material 6a by moving the rams2a and 3a together. During compression the axial linner force F3previously placed on the liner 4a is continued.

The liner 4a and the material 6a contained therein may be heated to anydesired temperature by the heating coil 12. The heat can be applied atany stage and continued throughout the operation if desired. Atherrnocouple or other temperature sensitive device (not shown) can beused to control and measure the temperature.

After maintaining the pressure and temperature at prescribed magnitudesand for prescribed times dependent on the material 6a to be compressedand the desired density of the compressed material, the pressure on thematerial 6a is released by moving the rams 2a and 3a apart. Thecompressed material 6a is then removed from the die 1a. The compressiveaxial liner force, F3, on the liner 4a may also be relased.

It has been yfound that the application of the axial liner force F3 onthe die liner 4a as described above permits much greater than normalforce to be applied to the material before comrpessive failure lof thedie liner 4a occurs. The reason for this, though not fully understood,is believed to be that the axial liner force F3 opposes the axialcomponents of the essentially radial force F2, thereby preventing theseparation of the liner as in the prior art method described above. Bypreventing this separation, or axial tensile failure, the catastrophiccombination of compressive and axial tensile failure of the die liner 4ais prevented.

Hence, by applying axial compression to a die liner 4a, the walls ofwhich are supported by a support sleeve 5a, material 6a within thatliner 4a can be compressed at pressures greater than normally possible.

The preferred material for the die liner 4a is graphite. This is atleast because of its lubricating quality, ease of machining, low cost,and relative chemical inertness under many conditions. Graphite is alsocapable of being used at very high temperatures, provided that theoperation is carried out in a vacuum, or in an inert atmosphere free ofoxygen and moisture. However, the general method described herein is notlimited to the use of graphite as a die liner. Any material that ispresently in use or may be used in the -future as a die liner can beamployed in the practice of the novel method of pressing materialsdescribed herein. Examples of materials other than graphite that may beused as a die liner are metal nitrides and carbides such as boronnitride or boron carbide; and ceramic oxide materials such as alumina,magnesia, and zirconia.

The support sleeve 5a, the rams 2a and 3a, and the plug 11 can be madeof any materials that are capable of withstanding the stresses uponthem. For example, the support sleeve 5a, rams 2a and 3a, and the plug11 may be made of essentially any of the materials used in theconventional presses and hot-presses as support sleeves or compressionrams, or of any materials that may be used therefor in the future.Examples of materials presently in use include molybdenum alloys,stainless steel, chromenickel alloys, and tungsten carbide.

Any of the conventional means for heating the die and material containedtherein may be used including resistance and induction heating of thedie liner or material therein.

The method is most applicable to the pressure compacting and pressuresintering of powders which melt at relatively high temperatures, e.g.,ceramics, ferrites, or other refractory materials. Compaction of suchpowders normally requires high pressures and temperatures. However, thegeneral method is applicable in any pressure compacting, pressuresintering, or presure molding proceess. Examples of materials that maybe compacted or pressure sintered by this method to form uniform compactbodies are given in the following table.

,4 Class of materials: Examples Semiconductive CdS, ZnS, CdTe, ZnSe,

compounds PbTe GaAs, GaSb, InSb. Elemental semicon- Ge, Si.

ductors Mn-Zn ferrite, Li-ferrite, Ferrites yttrium iron garnet. Oxideceramics A1203, Si02, BeO, ZpO2,

Mgofrioz, U02. Ferroelectrics BaTiO? PbZrO3, Pb (Zr,

T)O3, NaTaOg, NaNbOg. superconducting Nb-Sn alloys.

materials Refractory carbides and ZrC, HfC, BN, UC2

nitrides Other inorganic MgF2, LiF, CaFz, LaF3.

compounds Metals Ni, W, Ti.

The method is also applicable to pressure bonding of various materials,and to pressure molding of self-lubricating composites from metals andsolid lubricants. Typical ranges of pressure and temperature for thecompression of inorganic powders by the novel method are 10,000 to100,000 pounds per square inch pressure at temperatures of from aboutlroom temperature to about 2500 C.

Example 1.-A die and ramassembly of the type shown in FIG 2 having twocylindrical rams 2a and 3a of about 1/2 inch in diameter and graphiteliner 4a with a wall about 1%@ inch thick is employed. The plug isscrewed down with a wrench so as to apply a torque of 42 foot-pounds tothe plug 11. A value of about 17,000 p.s.i. of axial liner pressure iscalculated from this torque. The lower ram 2a is introduced into theliner 4a. A. measured quantity of gallium arsenide powder is then placedin the liner 4a. The upper ram 3a is introduced into the liner 4a andadvanced toward the lower ram 2a thereby compressing the powder betweenthe rams Za and 3a. The pressure is maintained for about one hour withthe powder heated to about 900 C. Then, the upper ram 3a is retractedfrom the liner 4m and the compressed material is removed from the liner4a. Ram pressures of at least up to abou-t 80,000 p.s.i. can be appliedusing the novel method. Prior methods using dies of the type shown inFIG l with the same liner 4a and ram 2a and 3a dimensions can applypressures to the powder of only about 25,000 p.s.i. at 900 C.

Example 2.-Using the die and ram assembly described in Example l andfollowing the same general procedure as described therein, calciumfluoride powder can be pressed with applied pressures of at least about50,000 p.s.i. at a temperature of about l,000 C. The prior methods usingdies of the type shown in FIG l having the same liner and ram dimensionscan only be used at applied pressures to the powder of up to about15,000 p.s.i. at l,000 C.

What is claimed is:

' 1. A method for pressing particulate material to form a compactedunitary body therefrom including the steps of:

(a) placing the material to be pressed into a die, said die comprising asupport sleeve and a liner within said sleeve, said support sleevesupporting said liner when outwardly directed radial pressure is presenton said liner;

f(b) compressing said liner in the axial direction;

y(c) compressing said material within said die so as to form a unitarybody therefrom while maintaining compression on said liner in the axialdirection of sufficient magnitude to prevent failure of said liner.

2. A method for pressing material as described in claim 1 includingheating the material to be compressed so that said material will be at aprescribed elevated temperature during the step of compressing saidmaterial.

3. A method for pressing material as described in claim 1 wherein saidliner is made of graphite.

4. A method for pressing material as described in claim 3 including thestep of removing essentially all of the oxygen and moisture from theatmosphere surrounding said material to be compressed.

5. A method for pressing materials as described in claim 3 wherein saidmaterials to be compressed are powders of inorganic compounds, saidcompounds being compressed at pressures in the range lbetween 10,000 and100,000 pounds per square inch, while at temperatures of from about roomtemperature to about 2500 C.

References Cited UNITED STATES PATENTS 6 2,909,417 10/1959 Osenberg51-293 3,230,286 1/1966 Bobrowsky 264-120 OTHER REFERENCES AdvancedMechanics of Materials, 2nd edition, F. B. Seely and 1.0. Smith, JohnWiley & Sons, New York, 1952, pages 301 and 309.

ROBERT F. WHITE, Primary Examiner.

I. R. HALL, Assistant Examiner.

U.S. Cl. X.R.

